INTERROGATING THE INTERPLAY OF ATMOSPHERIC PCO2 AND PO2, CLIMATE, AND LATE PALEOZOIC TROPICAL TERRESTRIAL ECOSYSTEMS USING MODEL-DATA INTEGRATION (Invited Presentation)

Earth’s earliest forests expanded throughout the late Paleozoic tropics undergoing repeated restructuring on the 10⁵- to 10⁶-yr timescales. Although it has long been established that these early vascular plants responded to climate, their ability to feedback on the environment is debated given inferred low photosynthetic and hydraulic capacities and an inability to respond rapidly to environmental conditions. Here, we interrogate the physiological functioning of dominant taxa of paleotropical ecosystems and evaluate the potential for thresholds of physiological functioning and for vegetation-climate feedbacks. We do so by applying taxa-specific leaf morphologic and geochemical measurements from well-preserved leaf fossils, time-specific atmospheric CO₂ and O₂, and site-specific meteorology to Paleo-BGC, a modified version of a process-based ecosystem model (Biome-BGC 4.2.1). Our simulations, constrained by fossil data, reconstruct the physiological functioning of the extinct plants by simulating their in vivo response to paleo-environmental conditions.

Model results identify physiological mechanisms capable of driving repeated replacement of wet-adapted plants by seasonally dry-adapted taxa, archived in paleobotanical records at the interglacial-glacial scale and long-term with progressive aridification. Modeled water-use and soil-water attributes of drought-adapted vegetation reveal their ecophysiological advantage over the iconic wetland plants (e.g., lycopsids and medullosan seed ferns). We further identify CO₂ (<400 ppm) and precipitation thresholds that lead to a loss of physiological viability, and when coupled with proxy pCO₂ estimates, provide a mechanism for the series of major and permanent turnovers in late Paleozoic tropical ecosystems. Although model results indicate a larger effect of pCO₂ than pO₂ on physiological functioning, elevated pO₂leads to overall reduced plant transpiration, increased water retention in soils, higher surface discharge rates, and increased nitrogen export. In turn, higher simulated soil-water retention and surface-runoff rates with long-term aridification and elevated pO₂ had the potential to strengthen CO₂—climate coupling during the late Paleozoic through increased soil erosion and weatherability.